Having trouble with a client site. After I changed

Having trouble with a client site. After I changed the width of her content area, she's saying there started being an issue with her galleries.

Not sure how, but happy to fix if y'all can help me figure it out!

Basically if you look at her gallery here: http://sadieraeandco.com/2013/10/23/redhead-halloween-costume-ideas/

It's a basic WP gallery using the "Square Tiles" setting, but WP is generating an extra div every 3 thumbnails. Anyone got any idea why?